Pretty simple suggestion, make it possible to define custom_book_item property in book.json as array of items rather than a single one.

It is already possible to open GUI of Patchouli books through API method, allowing it to be accessed through multiple items or even means other than items. However, it appears that based on this property Patchouli decides whether or not hold-Ctrl functionality should be available on items that are linked within the specific book, allowing to quickly switch to the book and open it right at the entry and page where item in question is linked. In my specific use case, I am introducing an item in Enigmatic Legacy that will serve as variation of the standart book item, which allows to access contents of the same Patchouli book but has improved melee abilities. I bet there migth be more cases out there.